Here is the full list of all 30 Serious Sam Double D XXL achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Kill over 50 Gnaar in a single level
Create your first 6 gun stack
Find 10 Secrets
Find 30 Secrets
Find ALL Secrets
Rescue 3 dinosaurs by destroying their helmets
Beat any level in under 2 minutes in its entirety
Kill a Red Biomechanoid with only chainsaws
Beat the game on any difficulty
Complete a level from the start at over 100% gameplay speed
Find 16 guns
Find ALL guns
Find all 4 of any gun type
Unlock 2 golden guns
Unlock ALL golden guns
Clear 2 Challenge levels
Clear 8 Challenge levels
Clear ALL Challenge levels
Beat any level without dying
Beat any level without getting hurt
Replay any previous level (Choose Level menu)
Find Mental's Secret Closet
Find the Secret Red Light District
Find the Monument to Parallax
Kill 10 Gnaar on ceiling spikes in Amethystine Depths
Help Templeton unlock his secret room in Amethystine Depths
Eat one Boston Creme or Cannoli
Find the Jump Pad
Kill a Giant Flea in Fireball Fury
Find or reach 100% levels, challenges, secrets, guns, and connectors